Ran my rebuilt engine for about 200 miles. It ran fine, sounded good and plenty of power, but started to smoke on right cylinder pretty bad. Did a compression test and right cylinder was down compared to left. 60psi on first kick on right, 90psi first kick on left. pulled the head and found this!

pulled the barrel and found both circlips on the wrist pin missing! I soon found most of them when I took the sump off though ! Well chewed and in bits. I was so bloody sure I put them on correctly. Obviously not as correctly as I should!

Anyway, to questions:

1. Apart from the obvious rebore new pistons and rings, can I get away with cleaning out oil tank, fresh oil and off we go? I really do not want to pull this engine again and split the cases. I stuck a magnet down to bottom of crank and found nothing.

2. Thinking I should maybe pull the oil tank as well and flush it out?

If you have a return line filter before your tee to the top end, you should be OK cleaning out the sump. The only item in danger is the return side on your pump. That won't put your engine in danger just potential wetsumping.

I was pretty sure they snapped into the grooves, double checked them. Not good enough; they came out. Looks like the wrist pin was "floating" left and right and made a right mess of the cylinder at the top and bottom travel of the stroke, plus lesser scoring up the cylinder wall. Hoping I don't have to take it more than 20 thou over, but might be 40. Lucky it is a standard barrel so it is recoverable.

I don't have a filter on the bike. Sort of depressingly resigned to maybe stripping the damn engine down. But then wondering if it is needed. no big bits will have got onto any bearing surface? there were plenty of broken bits in the sump, no longer than 2-3mm, but of course I don't know if all the bits are there from both circlips.

I really don't want to pile it on you but think about this for a moment...you say BOTH clips came out of one piston. Have you looked at the piston carefully? Perhaps the locating grove was never cut, and you simply installed the clips in the place where the groove should have been?

The other scenario I can see is a miss alignment of the bore to the rod. Have you ever measured the rod centers to each other while installed in the engine cases? End play on the crank? Bore centers to rod centers?

These are all easy processes to measure, and they are critical numbers to have.

Adding to NickL's list are the circlip grooves deep enough, not that you can do much about that for most peoples workshops,does the wrist pin have a decent chamfer each end?I always remove any tails on wire type circlips if they have them and make a vertical groove at the wrist pin hole to enable the tails to be dug out with a scriber. With a good chamfer on the end of the wrist pin, any sideways force on the wrist pin will have a locking effect on the id of the circlip. "It should" prevent the circlip being popped out which is more likely if the pin is pretty much square ended. You can't do this with the flat sided circlips with eyes in them but with their square circlip groove don't seem so prone to this type of failure, they just let go altogether!

thanks for all the tips on checking alignment, measurements etc. When I put it all together the rods, little ends etc all seemed OK, but then it was the first time I have done this so maybe missed something.

I have not removed the pistons yet, but there seems to be a groove for the circlips. I will have a closer look when I have them removed. Got to figure out now how to remove the wrist pins!

In a perfect World the wrist pin clip (aka circlip) is not under any side load from the wrist pin. But we can easily introduce a situation where the circlip is under a pounding side load. This happens when the wrist pin bushing, and thus the wrist pin itself, is not square to the crankshaft center-line. With the pin not being square to the crankshaft with each successive up and then down movement of the rod the pin is first pushed to one side and then the other. The repetitive movement of the pin puts a lot of pressure on the circlip.

Now some movement is expected and that is why wrist pins designed to be used with wire circlips have their ends chamfered. As mentioned this pushes the circlip into the groove rather than trying to collapse it and out the end.

But when we expereince this type of failure you expect the area adjacent to, and just outside of, the circlip groove to shows signs of distress. It is usually the area just outside of the circlip groove that fails allowing the circlip to be caught in the pin recess on the side of the piston. Here the up-and-down movement allows the circlip to eroded a lot of the aluminum around the circlip groove away. In the process the circlip is usually broken into pieces.

Looking at the sides of the pistons can tell you a lot about what happened.

-----When you get a new set of pistons examine the circlips. If wire then make sure the circlip groove in the piston is round. If flat (seeger clip) the make sure the groove is cut square. NEVER mix circlip types!

In over 50 years of building bike engines I have had only one circlip failure and it was MY FAULT. It was on an BSA A10 owned by Nathan Nichols (Nichols lathes)in 1968. I had replaced the wrist pin bushings and failed to check to see if they were installed properly. Failing all sorts of measuring equipment one can put a pair of steel parallel bars across the crankcase mouth, put the pin in the rod and lower the pin to the parallels, and checking with a feeler gage to see if the pin sits firmly on both bars (This is not how I would check rods if I was to use them racing!!).

Unlike Pete R, where I seldom disagree, I have not had any experiences where I have seen a failure with a wire ciclip with a tang without also a wrist pin bushing that was not square to the crankshaft. In these situations a standard circlip would have come to the same end. For the past 30 plus years JCC pistons have come with wire circlips with a tang Yes, the technical literature explains that even how you locate a standard wire circlip can be a factor at high rpm's the threshold for such a failure would be at rpm's far exceeding anything one of the engines would see.John

I shall examine the piston when I get it off. I used the circlips that came with the pistons (and wrist pins) so I would hope they were of the correct type. I will also check the little end bushes using the two straight edges as you suggest. I don't want the same thing happening on the rebored cylinders.

Surprised that no one has tangentially mentioned the goodness or badness of nylon buttons instead of circlips in this application? I know that some folks use them; is this an item of modern goodness that might be better than potentially brittle wire here, or are they fraught with more problems than they are worth?

Surprised that no one has tangentially mentioned the goodness or badness of nylon buttons instead of circlips in this application? I know that some folks use them; is this an item of modern goodness that might be better than potentially brittle wire here, or are they fraught with more problems than they are worth?

Lannis

Can you expand on this? are you meaning something like a nylon plug that goes in the piston? Where do you get them, and how do you size them?

Surprised that no one has tangentially mentioned the goodness or badness of nylon buttons instead of circlips in this application? I know that some folks use them; is this an item of modern goodness that might be better than potentially brittle wire here, or are they fraught with more problems than they are worth?

The newer Teflon buttons will out last the rings in many cases. They began using these in race engines about 10 years ago that I know of.

I have only used one set in a 530ci GM big block built for offshore racing where the engine was run at or near WOT for hours at a time (ran the Around the Island (Long Island, N.Y.) race 2 years consecutive). These were JE pistons, and Teflon retainers. The retainers had a 1/8" hole drilled in the centers, and it was in the instruction set to be SURE the holes were clear to allow for air transfer thru the pins.

I ran that engine for 2 full seasons in New York, went thru four sets of rings, two sets of crank, and main bearings, and the retainers were original the entire time.

I sold the boat/motor (21 foot Monza, after the second season, and lost track of it from there.

When I have the old piston and wrist pin removed I am going to do as John Healey suggests and place two straight edges on the crank case and see if the pin is true with it in position in the little end.

Question is, how much out of whack in thou from one side to the other is acceptable?

Any measurable difference will create a "walking" issue.If you measure any difference at all the next step is to remove the bushes, and measure the rod ends for square.

If the ends are still measuring "off" the next step is remove the rods, and check if they are square. After that the big ends, and bearings, and finally the crank itself.

If you see BOTH small ends have a measurable difference on the same side (with no bushes installed), I would go straight to the crank, and the cases.

Edit: You may see .002 play on BOTH sides. That would be the absolute maximum play you could run. It must be both sides tho. I've seen numbers up to .006 but never felt comfortable with that. Perhaps the Brits. are different but all else applies.

If I was trying to estimate parallel on rod eyes, as viewed front to rear, I wouldn't expect great accuracy over the length of a gudgeon pin.It's still better than nothing.

I would start by using a 0.015" feeler under one side of the pin (and pushing hard down on the other side of the pin) and then measure the clearance under the other side of the pin with another feeler. Then swap the feelers side to side, and see if readings are the same. If, for example, you readings were 0.015" and 0.005" in both cases, you'd be OK.

Got the pistons off and I found I only lost one complete cir clip. the other one was still in place, but the tang had gone. The wrist pins seem too sloppy in the little end bushes, probably around 10 thou rocking by feel which is out of spec according to the manual. The wrist pins were 2.357" long with the manual stating 2.368 minimum so maybe that had something to do with the problem. The wrist pin could wander back and forth?

wrist pin diameters measured on the calliper were .749 with the manual stating .75 min. As best I could measure with callipers the bushes were .754 right and .752 left so I reckon I am in for a tear down and get new bushes fitted? Not a job for me and while it is in bits I will get the shop to check the bottom end as well.

Thanks for all the help and advice on here, much appreciated. Looks like it is off the road for the rest of our short riding season. Good job I have a modern beast to ride

On the bright side, once you rebuild her you have another 30 years of service, and smiles to look forward to.

If the small ends were that sloppy it's almost a given the big ends need attention. I would 100% tear it down, find all the missing metal, and rebuild it correctly. I'd rather ride a known good bike than worry to death what was going to cut loose next.